The recent portable electronic devices such as a folding-type mobile phone and the like use a flex-rigid multilayer wiring board. FIG. 22 shows an example of such a conventional wiring board, in which inflexible rigid portions 500 and 520 are connected to a flexible portion 510 via a flexible substrate 544. Normally in the rigid portion 500, the flexible substrate 544 and pattern layers 504 and 506 on the surfaces of the rigid portions 500 and 520 are electrically connected to each other via a conductive layer in a plated through-hole 502 (cf. Japanese Patent Application Laid Open No. 90756 of 1993).
The conventional flex-rigid multilayer wiring board according to the invention disclosed in the Japanese Patent Application Laid Open No. 90756 of 1993 is manufactured by stacking a flexible substrate formed from a highly bendable base material such as polyimide resin film onto a rigid substrate formed from a rigid base material such as glass polyimide resin and having a circuit formed thereon with a prepreg, adhesive sheet or the like being inserted between them; stacking another such rigid substrate onto the flexible substrate with another such prepreg or adhesive sheet being inserted between them, joining them to one another by thermo compression bonding; and then subjecting a stack thus formed to many processes such as drilling, through-hole plating, resist coating and etching.
Since the flexible substrate should be freely bendable, so it uses no reinforcing material such as paper, glass fiber or the like used in the rigid substrate but it is formed from a substrate material including a highly bendable thin base film of polyimide resin, polyester resin or the like as an insulative material and a flexible copper foil attached to the base film.
When used singly, the polyimide film forming the base film can resist a temperature of 400° C. or more and also a soldering temperature of 250° C. or more satisfactorily during mounting of parts. In addition, it keeps a stable performance even if the environment varies after the printed wiring board using the polyimide film is actually assembled in an electronic apparatus. Therefore, the polyimide film is used in overwhelmingly more cases than the polyester film.
Also, the coverlay to protect a conductor circuit formed by etching a flexible copper foil attached to the base film uses an adhesive-coated polyimide film in many cases because of its flexibility.
The current social need is that the portable electronic devices should be higher in functionality and packaging density of parts. To meet this need, it has been demanded that the parts installed in the electronic devices should be designed smaller and correspondingly the wiring width of the circuit board on which such parts are to be mounted should be reduced more.
Especially, it is demanded that the wiring patterns formed on the flexible substrate included in a bendable flex-rigid printed wiring board should be finer and denser (the number of wires per unit area should be increased).
However, the polyimide film used as insulative base material in the flexible substrate is highly water-absorbing and largely varies in dimension. Because of these properties of the polyimide film, the land should be preformed large in size and the workpiece be formed small for an improved accuracy of alignment. Therefore, the polyimide film could not assure any high connection reliability and could not show good results in the thermo cycle test or similar reliability test.
Also, the film used as the insulative base material in the flexible substrate above is just a film formed from the polyimide resin, not any film formed from a core having the polyimide resin impregnated in the core. Therefore, the polyimide film is disadvantageous in the following:
(1) It has no sufficient strength;
(2) Bending cannot be made with a constant radius of curvature; and
(3) Because of the problems (1) and (2) above, the flexible substrate is easily deformed around a portion thereof where it is to be bent or the conductor circuit is broken. Especially, in case the flexible substrate is repeatedly bent or the conductor circuit is formed from fine wiring patterns, the above problems will be noticeably serious.
Also, a waving will possibly take place at the bending portion of the flexible substrate and near an area where the rigid portion is formed. When the waving is formed, it will possibly result in cracking at a portion of the substrate where the waving has taken place or in breakage of the conductor circuit.
Further, forming of a cover lay to protect a conductor circuit provided on the flexible substrate from an adhesive-coated polyimide film will lead to deterioration of the connection reliability and insulation reliability of the through-hole. Therefore, drilling, desmearing, plating and the like should be effected under specially managed conditions, and combination of the flexible substrate with any other material will be limited.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the related art by providing a flex-rigid printed wiring board that can be bent largely while keeping a sufficient, constant strength at a portion thereof where it is to be bent, and a manufacturing method for the flex-rigid printed wiring board.
Also, the present invention has another object to provide a flex-rigid printed wiring board excellent in connection reliability and in which it is possible to prevent deformation of a flexible substrate at a portion where the substrate is to be bent, breakage or waving of a conductor circuit and a manufacturing method for the flex-rigid printed wiring board.